Displays based on organic electroluminescent devices such as organic light-emitting diodes (OLEDs) have the advantages of self-emission, fast response, wide viewing angle, high brightness, colorfulness, lightness, and the like over liquid crystal displays (LCDs), and are regarded as promising display products.
The film formation of an organic electroluminescent device generally includes an evaporation process and a solution process. The evaporation process has been widely used in the manufacture of small-size display panels. The solution processes may include inkjet printing, nozzle coating, spin coating, screen printing, and the like, wherein inkjet printing is considered an important way for manufacturing large-size display panels. The ejection volumes of the nozzles in an inkjet head are not exactly the same. If the difference in ejection volume between nozzles is larger than ±0.3%, then uneven mura of the resulting pixels would be obtained that is perceivable by human eyes. Therefore, test printing needs to be performed before an actual print process to accurately calibrate the ejection volume of each of the nozzles in the inkjet head.
It is however time-consuming to calibrate the ejection volumes of all the nozzles in one or more inkjet heads to be consistent or approximately consistent. For example, each of the inkjet heads is typically equipped with 256 or 512 nozzles arranged in a straight line. For 3840 pixels, 15 inkjet heads are required if an inkjet head with 256 nozzles is used. Typically, it will take 3-5 hours to calibrate the ejection volume error of 256 nozzles within 0.3%. Therefore, the calibration of 3840 nozzles will be quite troublesome.
There are already some improved solutions. For example, a plurality of print processes (hereinafter referred to as “multi-printing scheme”) are performed on the same pixel with a head having smaller ejection volumes. That is, each of the pixels is formed of ink droplets from a plurality of nozzles, each nozzle having a different predetermined ejection volume of such as large, medium, or small size. This method may reduce the probability of occurrence of the mura to some extent because the volume error of the ink droplets is averaged. The more the number of ink droplets used to form the same pixel, the better the averaging effect. However, there is a problem with this approach. First, there is a certain probability of mura exacerbation in multi-nozzle printing. For example, for 2-nozzle printing, the probability of mura exacerbation is 2×½2=½; for 4-nozzle printing, the probability of mura exacerbation is 2×½4=⅛; for 9-nozzle printing, the probability of mura exacerbation is 2×½9= 1/256. That is, at least 9 print processes per pixel are required for 256 pixels to avoid mura exacerbation. Second, the increased number of prints results in increased print times and extended production beats. Furthermore, ink droplets stay on the substrate for a longer period of time, resulting in reduced process stability.